The present invention relates to a circuit for preventing overcharge and overdischarge of secondary batteries.
There has been known heretofore the following method as a preventive measure against overcharge and overdischarge of a secondary battery. The conventional measure for prevention of any overcharge that may be caused by a charger is carried out by controlling the charge terminal voltage of a battery during the charging operation, and such means is sufficiently effective for a single battery or parallel-connected batteries. However, it is generally frequent that batteries are used in a series-connected state. And in such a case, it is impossible to control the voltage of the individual battery although the series-connection terminal voltage can be controlled. Therefore, when at least one of the series-connected batteries is short-circuited, the other batteries are overcharged during the charging operation, and thus the above method fails to achieve complete prevention of overcharge. Another overcharge preventive measure is based on utilization of a phenomenon that the internal pressure of a battery rises upon occurrence of overcharge, and it is carried out by mechanically cutting the relevant current leadwire to interrupt the charge current. The purpose of this method is not to prevent overcharge itself but to prevent breakdown of the battery that may result from an abnormal rise of the battery temperature or high internal pressure caused with progress of the overcharge. Once the current interruption is executed, the relevant battery is rendered nonusable.
Meanwhile with regard to prevention of overdischarge, there is known a method of selecting a suitable metal for a negative-electrode current collector whose dissolution voltage is possibly as low as zero. For example, the use of nickel instead of copper is somewhat effective but is not completely satisfactory. Particularly in series-connected batteries, over discharge may be unavoidable in one battery due to the difference existing among the individual batteries, and therefore the life of the charge/discharge cycle is extremely deteriorated.
In addition to the above, there is also known the following conventional method as an overcharge/overdischarge preventive measure for nonaqueous secondary batteries.
FIG. 8 is an exemplary constitution of a nonaqueous secondary battery such as a lithium ion type.
In this example, a positive electrode is composed of LiCoO.sub.2 6 as an active material, while a negative electrode 7 is composed of carbon of a graphite structure as an active material. Such active material is held by an aluminum current collector 8 in the positive electrode or by a copper current collector 9 in the negative electrode 7.
The positive and negative active materials are disposed opposite to each other through a separator 10, and the space between the two active materials is filled with an organic electrolyte 11. In the charge and discharge characteristics of this battery, as graphically shown in FIGS. 9 and 10, a close correlation exists between the charge/discharge energy and the battery terminal voltage.
The battery has a design voltage determined by the component materials of the battery and the design thereof, and an action of charging beyond such voltage is termed overcharge. As a result of overcharge, there occur (1) deposition of the lithium metal on the negative electrode, (2) decomposition of the positive-electrode active material, and deposition of the cobalt metal or cobalt compound on the negative electrode due to cobalt ions derived from the decomposition, and (3) decomposition of the organic electrolyte.
Such deposition of the lithium metal, cobalt metal or cobalt compound causes short-circuiting of the positive and negative electrodes, and the decomposition of the positive-electrode active material or the organic electrolyte induces extreme deterioration of the battery. It is therefore impossible to ensure sufficient reliability of the battery unless overcharge is essentially averted.
If a charged battery is discharged with an external load connected thereto, the battery voltage is lowered and, in accordance with continuous discharge, the battery voltage comes to reach the dissolution voltage of the negative-electrode current collector (copper). A further discharging action subsequent to transgression of the dissolution voltage is termed overdischarge. Upon occurrence of such overdischarge, naturally the copper is ionized and liquated into the electrolyte. The dissolution of the collector metal causes deterioration of the current collecting function and fall-off of the negative-electrode active material to consequently reduce the battery capacity. Furthermore, the copper ions thus liquated are deposited abnormally on the negative electrode at the next charging to cause a short-circuiting fault of the positive and negative electrodes. Therefore, it is essentially necessary to avert such overdischarge as well.
As an overcharge/discharge preventive measure for the above-described secondary battery, there is proposed in Japanese patent application No. 03-97734 filed Apr. 26, 1991 a method which utilizes the correlation between the voltage of such secondary battery and the charge/discharge energy thereof by a procedure of continuously detecting the terminal voltage of series-connected batteries and interrupting the charge or discharge current at a predetermined voltage above the design battery voltage or below the dissolution voltage of the negative-electrode current collector metal, thereby preventing any overcharge or overdischarge of each battery to ensure the reliability and safety thereof.
FIG. 11 shows a fundamental arrangement to carry out the overcharge/overdischarge preventive method mentioned above. In this diagram, four batteries are connected in series and parallel to one another. Voltage detectors 12, 13 detect the terminal voltages of the batteries in series-parallel connection and turn off a switch 14 at any terminal voltage above the design voltage to thereby prevent overcharge. Meanwhile, a voltage detector 15 detects the terminal voltage of the series-connected batteries and turns off a switch 16 at any terminal voltage as below the design voltage determined by the metal of the negative-electrode current collector or the cutoff voltage of an apparatus which uses such batteries, thereby preventing overdischarge.
However, there still remains the following problem even with adoption of the preventive measure described. Although the above exemplary arrangement is capable of detecting overcharge of an individual battery or overdischarge of two series-connected batteries, it is impossible to detect any unbalanced charge or discharge of the two batteries during simultaneously charging or discharging of the series-connected batteries. And there may occur an unbalance between the charged or discharged states of the two batteries in such a manner that one battery has been charged to the desired set voltage while the other battery has not yet been charged to the set voltage, or one battery has been discharged to the set voltage while the other battery has not yet been discharged to the set voltage. Particularly in lithium secondary batteries, it is absolutely necessary to avert any overcharge or overdischarge. Therefore, when one of the batteries has been fully charged in the aforementioned conventional circuit, the other battery fails to be fully charged even though overcharge can be averted by turning off the switch, hence inducing an unbalance between the charged states of the two batteries.